1. Field of the Invention
The present invention relates to a multiplexer and in particular to a service multiplexer for the multiplexing of data and other signals from a plurality of signal sources for feeding to a public switched communication network.
2. Related Art
A service multiplexer is typically located in a customer""s premises and is used to deliver and receive various data services via a single line between the multiplexer and the public network. The signal sources may comprise a combination of voice, video and data services such as Ethernet connections.
The prior art multiplexer has typically been connected to the public network via a link having a fixed datacarrying capacity or bandwidth. Correspondingly the signal sources connected to the input of the multiplexer have been allocated a fixed share of that bandwidth.
With the advent of more sophisticated connections to the public network such as an available bit rate (ABR) connection in which the bandwidth of the connection to the public network is variable, it has become necessary to provide a more sophisticated service multiplexer which is able to accommodate changes in the bandwidth of the connection to the public network
According to a first aspect of the present invention there is provided a service multiplexer having a plurality of inputs for connection to a respective plurality of signal sources and an output for connection to a signal receiving system, the multiplexer being operable to transfer data from the inputs to the output for transmission into the signal receiving system as a synchronously transferred data cells, wherein the multiplexer includes bandwidth control means for receiving a bandwidth control signal from the signal receiving system representative of an available bandwidth for the transmission of the cells into the signal receiving system and activity detection means for monitoring the bandwidth of data received at at least one of the respective inputs, the bandwidth control means being operable automatically to adjust the bandwidth of data transferred to the output according to the available bandwidth and to the monitored bandwidth or bandwidths.
By monitoring the bandwidth of data received at at least one of the respective inputs, the service multiplexer is able to adjust the relative bandwidth allocated to each of the signal sources to make the most efficient use of the bandwidth available on the connection to the signal receiving system.
The bandwidth control means is preferably operable to adjust the rate of transmission of the data transferred by generating a feedback control signal for transmission to a signal source via a respective input. When directed to an appropriate signal source, such a feedback signal causes the respective signal source to alter its transmission data bandwidth. In this way the bandwidth control means is able to control the bandwidth of data entering the multiplexer to suit the bandwidth available in a signal receiving system operating on an ABR basis, i.e. which indicates available bit rates to signal sources.
For signal sources which are not capable of interpreting feedback control signals or as an alternative way to control the bandwidth of data which has been received at the inputs, the bandwidth control means may be operable to adjust the data of bandwidth transferred by discarding a portion of data received at a respective input.
Preferably, where the data received at an input is divided into a plurality of groups, any discarded portion of the data is chosen from a subset of the groups. Thus if the data being received at the input is, for example, constructed from a plurality of frames each constructed from a plurality of cells, the bandwidth control means may discard or delete cells only from a single frame or subset of frames thereby maintaining the data integrity of the remaining frames. In this way, the need for re-transmitting data is typically reduced since not all frames will have been affected by the cell deletion as would be the case when using a random, xe2x80x9cunintelligentxe2x80x9d cell deletion technique.
The multiplexer preferably includes a stored table for containing characteristic data about each signal source which includes an identification of which signal sources are operable to receive feedback control signals. In this way, the bandwidth control means can choose which technique to use to control the bandwidth of data being received at the inputs (and thereby to adjust the bandwidth of data transferred to the output) according to which control method or control methods may be used with a particular signal source.
Preferably the activity detection means is operable to recognise a new active signal source, for example, by recognising that data has started to be received from this new signal source and the bandwidth control means is preferably operable in this case, to change the distribution of bandwidth allocations amongst the signal sources by reallocating the bandwidth of at least one existing active signal source to permit the allocation of bandwidth to the new active signal source. Alternatively, assuming that the fill bandwidth allocated to the connection to the ABR-type receiving system is not being used, the bandwidth control means may simply allocate some spare bandwidth to the new active signal source.
The activity detection means may also be operable to recognise a signal source which has not transmitted data for a predetermined period of time and may therefore reasonably be considered to have stopped transmitting. In this case, the bandwidth control means is preferably operable to de-allocate the bandwidth allocated to such a quiet signal source and if necessary to reallocate the de-allocated bandwidth to other signal sources.
The activity detection means is preferably operable to recognise a signal source which is transmitting data at a bandwidth less than a predetermined proportion of the bandwidth allocated to that signal source for longer than a predetermined time period and in this case the bandwidth control means is preferably operable to reduce the bandwidth allocation of such a recognised signal source. Thus a signal source which has been allocated more bandwidth than it needs to use can automatically have the unused portion of bandwidth reallocated by the bandwidth control means to more needy signal sources.
The activity detection means may be operable to recognise a signal source which is transmitting at more than a predetermined proportion of its allocated bandwidth for longer than a predetermined time period in which case the bandwidth control means may be operable to allocate more bandwidth to such a recognised signal source. In this way, if the activity detection means recognises that a signal source is likely to be able to use a larger bandwidth, the bandwidth control means may allocate more bandwidth to that signal source which may, for example, be spare bandwidth which is not being used or which may result from a reallocation of bandwidth to other signal sources.
Preferably, the multiplexer is arranged to provide separate data paths for the transfer of data, for signal sources which respectively are operable to receive feedback control signals and are not operable to receive feedback control signals. Since those signal sources which are not operable to receive feedback control signals are likely to be subject to cell deletion or deletion of portions of data received at the input in some other way, it is preferable to separate within the multiplexer the data paths of those signal sources which will have data deleted from the data paths of those which will not. In this way, those signal sources which are operable to receive feedback control signals and which can be commanded by the multiplexer to reduce their transmission bandwidth are not affected by cell or data deletion in addition to receiving reduced bandwidth feedback control signals.
According to a second aspect of this invention, there is provided a method of operating a service multiplexer having a plurality of inputs for connection to a respective plurality of signal sources and an output for connection to an ABR-type signal receiving system, the method comprising transferring data from the inputs to the output for transmission into the signal receiving system as a synchronously transferred data cells, receiving a bandwidth control signal from the signal receiving system representative of an available bandwidth for the transmission of the cells into the signal receiving system, monitoring the bandwidth of data received at at least one of the respective inputs, and automatically adjusting the bandwidth of data transferred to the output according to the available bandwidth and the monitored bandwidth or bandwidths, the method further comprising generating a feedback control signal to at least one of the signal sources to cause the said signal source to alter its transmission data bandwidth.
It will be understood, at least in the context of this patent application, that a signal source may be a single physical source or may be one of a plurality of virtual channels (which may themselves be grouped into one or more virtual paths) sourced from one or more physical sources. The terms xe2x80x9csignal sourcesxe2x80x9d and xe2x80x9cinputsxe2x80x9d should therefore be construed accordingly.
The invention will now be described by way of example with reference to the drawings in which: